How whichsidetosit works: the method behind the verdict

When this site tells you "sit on the left," that answer comes out of a deterministic calculation, not a database of opinions. This page documents the method in full - partly for transparency, partly because the method is the interesting bit. If you can follow a compass, you can follow all of it.

Step 1: The flight path

We model your flight as a great circle - the shortest path between two points on a sphere, which is what airlines actually approximate. The path is sampled at 100 points, and each point carries the aircraft's instantaneous compass heading, because on long routes the heading changes continuously (New York–London starts out heading north-east and lands heading south-east). Real flights deviate for weather and airways, typically by a few tens of kilometres - enough to matter for spotting a specific building, not enough to flip which side a mountain range or the sun is on.

Step 2: The clock

You give us a departure date and local time. We convert that to universal time using the origin airport's IANA timezone - daylight-saving-aware, which matters more than you'd think (a "6 pm" departure is a different sun by an hour depending on the season). Cruise speed and climb/descent profiles then stamp every sampled point with an estimated time, and each point is assigned a phase: takeoff, cruise, or landing.

Step 3: The sun

For every timed point we compute the sun's azimuth (compass bearing) and altitude using standard astronomical formulas. From this we derive: whether it's day, golden hour, twilight or night at each moment; whether the flight crosses a sunrise or sunset mid-air; and where sustained low-angle sun will glare into one side's windows.

Step 4: The landmarks

A static database of roughly 15,000 sights - mountain ranges and peaks, volcanoes, lakes, waterfalls, glaciers, deserts, canyons, national parks, islands, coastlines, UNESCO World Heritage Sites and major cities - built from public sources (Wikidata, GeoNames, OurAirports, Natural Earth) plus a hand-curated list of marquee sights. For each one near your path we ask: at closest approach, is its bearing left or right of the aircraft's nose, is it within plausible viewing range for its size, and is it actually visible in the current light? Mountains vanish at night; city lights don't - so after dark the engine suppresses scenery but keeps cities, and it knows the difference.

Step 5: The verdict

Every sight and sun event becomes a scored vote for a side within its phase. When sides disagree, ranking applies: sunrise/sunset beats named landmarks beats general terrain beats glare-avoidance. The result is a per-phase pick (takeoff / cruise / landing), an overall headline, and a confidence level. Genuine ties are reported as a toss-up - a forced answer would be less honest than "either side works."

What we deliberately don't claim

Weather. We don't forecast cloud. A verdict is "this side, if it's visible," never "you will see this."
Exact routings. Air-traffic control, jet streams and weather move real tracks around the great circle. Sides are robust to this; precise photo timings are not.
Airline schedules. We estimate flight duration from distance and typical speeds. Your actual departure time is the input that matters - use the one on your ticket.

Try it

The whole pipeline runs in under a second for any pair of the ~9,000 airports with scheduled service. Enter a flight, or read the geometry guide if you'd rather learn to do the sun half in your head.

More guides

Or just check your own flight - enter a route, date and departure time and get a left/right verdict in a second.